Regulatory bodies set out detailed specifications for cyclic keying of radio transmitters, for example in mobile radios, in order to minimise spurious emissions.
International patent application PCT/EP91/00146 describes an arrangement in which a power rise and fall envelope is defined in a read-only memory (ROM) in order to provide a highly controllable rise and fall power envelope. In the arrangement described in that document, there is a control loop in which the output power of the power amplifier is detected in a detector and fed in a feed back loop to one input of a comparator regulator which controls the power amplifier. The other input of the regulator is derived from a digital-to-analog converter, which derives its input from the ROM.
A problem with prior art power control loops arises from the dynamic range of the detector. Typically the amplifier has to be keyed up and down through 60 dB or more of dynamic range and typically a detector has no more than 30 dB of dynamic range capability. This means that until the power amplifier is keyed to within the dynamic range of the detector, the control loop is in effect open circuit. Moreover, the regulator typically requires a high gain in order to achieve satisfactory steady state operation. In the situation of having an effectively open loop circuit with a high gain regulator, the input to the power amplifier rises at a high rate in a virtually uncontrolled manner. This introduces a significant risk of over-swing. In effect, the input has commenced to rise too fast and too far before the output of the power amplifier enters the dynamic range of the detector and the control loop comes into operation.
In prior art arrangements such as that described in PCT/EP91/00146, a capacitor is introduced between the output and input of the regulator to provide an integration function. This is not always a fully satisfactory arrangement, because if the capacitor is too small there is over-swing and if it is too large there is a tendency to oscillate in the steady state.
There is a need for an improved power amplifier control arrangement.